Not Applicable
This invention relates to the design features of a hollow-core structure, and more particularly to the design features for a stitched carbon fiber reinforced hollow core fan blade for a turbofan engine.
In a continuous development cycle to improve turbofan engine operating efficiencies for jet aircraft, engine manufacturers have been designing increasingly higher thrust engines. This new generation of engines, known as xe2x80x9chigh bypass enginesxe2x80x9d or xe2x80x9cvery high bypass enginesxe2x80x9d, typically operate at significantly higher bypass ratios than their predecessors. To achieve these higher bypass ratios, airflow into the engine must be substantially increased. Generally, this is accomplished by increasing the inlet diameter of the engine. While this effectively increases the operating performance of the engine, it also requires that the first stage engine fan blades operate in a more demanding structural environment, as they increase in length and size. As a result, blade designs that worked in past applications are no longer able to meet these more difficult design parameters. Increases in blade size and mass result in dynamic-induced load conditions that can exceed the strength capability of current high-performance materials, such as titanium alloys or carbon fiber composites. Therefore, design changes in the structural geometry or shape of the blade are necessary to meet these new design requirements. While much work has been done on changing the exterior shape of fan blades by employing a wide-chord design philosophy, very little has been done to change the interior arrangement of the blade. One of the most effective ways to improve the structural performance of a rotating component is to reduce the structural mass of the part. This results in lower rotational inertia forces which reduces the internal loads experienced along the length of the blade. For rotating equipment where the rotational velocity is held constant and material properties are generally fixed, a reduction in mass is the single most effective way of accomplishing significant improvements in structural performance.
Reductions in part mass are especially beneficial because they produce a compounding-effect. As the mass is reduced, the magnitude of the internal loads drops, which in turn leads to further reductions in structural mass. A practical limit for this iteration is reached when the part reaches a fully stressed design point for a given set of material properties and load conditions. Once this design point is reached, the structural geometry or shape of the part is optimum. Any further improvements in the overall structural performance can only be achieved by changing or improving the material properties of the as-fabricated part. Thus it is important to note, that its geometric shape and specific material properties primarily influence the structural efficiency of a given component. Once the part geometry is optimized, only improvements in the strength and stiffness properties of the material will result in improvements in the structural performance. The premise of optimizing both the structural geometry and material properties is the overall basis for the stitched-composite hollow-core fan blade design described herein. It combines the superior mechanical properties found in carbon fiber based stitched composite materials with the optimum structural geometry inherent in a hollow-core structural arrangement. Through the combination of these two important design features, the optimum structural design point for a fan blade component is achieved.
Current state-of-the-art construction techniques for fan blade fabrication are comprised of hollow-core titanium designs and solid laminate carbon fiber designs. While both methodologies employ a wide-chord design geometry to improve the structural efficiency of the blade element, their respective design philosophies diverge based on their choice of materials. In each case, the design is driven by the limitations of their respective material systems rather than by the objectives of the design problem. For instance, the hollow-core titanium design approach uses the best structural geometry to optimize the internal loading through the structure, but the specific material properties (divided by density) of the titanium alloys are inferior to those of typical high-performance carbon fiber materials. In contrast, the carbon fiber design approach makes use of the high specific material properties of the carbon fibers, but does not take advantage of the hollow-core design approach to optimize the structural loading in the blade body. The optimum design approach would be to combine the hollow-core geometry with the high specific mechanical properties of the carbon fiber materials. The resulting blade design would provide the most structurally efficient fan blade possible for a given structural volume and set of design parameters.
While a hollow core carbon fiber fan blade is highly desirable, there are several limitations that have previously precluded its development. The primary concern for such a component is the limited damage tolerance behavior of laminated carbon fiber materials. As carbon fibers are known to be more brittle than metallic materials, meeting the impact damage design requirements for the bird-strike load condition has proved difficult. In order to meet these requirements, existing carbon fiber fan blade designs make two compromises: 1) blade sections are kept solid to maximize the amount of material at a given cross-section, and 2) the rotational velocity of the fan blade assembly is lowered to decrease the impact energy of the bird-strike. Each of these compromises is made because of the limited damage tolerance capability of the carbon fiber material systems and ultimately results in degraded engine performance. The key to removing these impediments is to improve the through-the-thickness mechanical strength of carbon fiber laminates. Prior efforts to accomplish this were focused on using toughened resin systems. While this approach has proven somewhat effective, the attained improvements are still significantly lower than the mechanical strengths realized in the primary fiber directions of the laminate. Since fiber properties are superior to than those of the resin, placing fibers in the z-direction (perpendicular to the lay-up plane) of the laminate offers the highest potential for improving the through-the-thickness mechanical properties of the carbon fiber material system. Indeed, the use of fiber reinforcement in the z-direction is the enabling advancement of the art that permits a hollow-core design approach to be successfully implemented using carbon fiber materials.
Another concern is the difficulty of fabricating a hollow core structure that is both damage tolerant and cost-effective using carbon fiber material systems. While several hollow core design approaches have been proposed in prior art, none of them is capable of satisfying the damage tolerance requirements necessary to meet the bird-strike load case because they rely solely on the resin interface to provide the through-the-thickness mechanical strength for the part. Since resin properties are not capable of providing an adequate level of damage resistance, further development of those particular fan blade design approaches has not resulted. Without significantly advancing the state-of-the-art in damage arrestment and residual strength, further development of carbon fiber hollow core fan blade concepts is unlikely.
To advance the state-of-the-art regarding damage tolerance, the stitched composite fan blade design described herein employs three key design features: 1) it uses through-the-thickness stitching to improve the z-direction mechanical strength, 2) it has a multi-element substructure design to provide structural redundancy, and 3) it has a continuous cover skin load path around the root section at each spar location. All of these advancements in the art were pursued to achieve the requisite level of damage tolerance necessary to make a composite hollow core fan blade operationally feasible. The out-of-plane mechanical properties are enhanced by the stitching and are no longer dictated by the inferior resin properties at the interface. The multi-element substructure arrangement not only reduces blade mass and internal loads it provides structural redundancy, whereby a single spar element failure does not preclude failure of the entire blade body. The continuous cover skin load path around the root section at each spar location maintains the load continuity in the highly loaded covers and reduces the structural fragility normally encountered at the root joint. All of these design features are extremely advantageous for the bird-strike load case because they work together to improve the damage characteristics of the materials as well as the overall load distribution within the structure. This will result in higher residual strengths and more resilient failure modes, as the redundant substructure elements redistribute internal loads to prevent the blade body from breaking-away during an impact. The synergy provided by these design features advance the art to the point where a hollow-core blade geometry made using carbon fiber composites is now feasible.
It is known that in prior art to design and manufacture hollow core titanium fan blades for large gas turbine engines by machining matching cavities in titanium plates, then diffusion bonding the halves together inside a die cavity. It is also known that laminating pre-plied carbon fibers together can produce a solid carbon fiber fan blade design. It is also know that hollow-core blade designs of pre-plied materials have also been proposed. However, the titanium-based designs and manufacturing methods do not utilize the high specific properties of composite materials, while the composite material-based approaches do not meet the rigorous damage tolerant requirements for the bird-strike load case. In either case, the resulting blades tend to be heavier than a fully optimized design would be. What is needed, therefore, is a reinforced composite hollow core fan blade design that can be cost-effectively manufactured and which also meets the more stringent damage tolerance requirements imposed by the design conditions of the high bypass ratio engine.
This invention solves the problem outlined above by combining the efficient load-carrying arrangement of a hollow-core multi-element substructure with the superior mechanical properties of stitched carbon fiber materials to produce a fan blade that is both highly damage tolerant and relatively inexpensive to manufacture. The blade is comprised of a multi-element substructure consisting of vertical spars and horizontal ribs that are enclosed within a cover skin, all of which is stitched together to form a dry preform assembly. Three titanium detail parts are added to the preform assembly to closeout the edges. The entire part assembly is infused with resin and cured to produce a rigid part. There are three important features that enable this design to be extremely durable and damage tolerant: 1the carbon fiber stacks that comprise the cover skin and substructure are stitched together to enhance the through-the-thickness mechanical properties, 2the substructure is a multi-element design to provide structural redundancy, and 3the cover skins are continuous at each spar location to provide improved load transfer around the root fitting joint.
Another important advantage of the invention is the hollow-core substructure design. It not only enhances the loading aspects of the part, it also permits the part to be manufactured using an internal pressure apparatus with a removable bladder system. The advantages of this are, smooth and uniform inner surfaces, as well as a significant reduction in part cost because it makes out-of-autoclave processing possible. The detail design nature of the substructure passage network is also a very important design features because it ultimately enables the fabrication of this invention.